Friday, May 5, 2017

Hanging Dr. Fong

Hanging Dr. Fong

By Andrew Ellis, NO6E

Long
before WB6IQN's latest article, “A Tri-Band Antenna Without Radials
For 2 Meters, 1.25 Meters and 70 Centimeters” in the March, 2017
issue of QST,
the remarkable Dr. Edison Fong has been designing inexpensive and
well-performing VHF and UHF antennas for ham operators. His DBJ-2
, a roll-up dual bander for 144/440 mHz appeared in the March 2007
issue, and the venerable DBJ-1,
the subject of this article, dates all the way back to QST of
February, 2003.

Ed's
antennas are easy enough to build, and he always shares all the
dimensions for those who want to construct them on their own. But
tuning the antennas is best done with a network analyzer, which most
of us don't have. As an alternative, Ed has some of his graduate
students at the University of California Santa Cruz Silicon Valley
campus build the antennas, which he sells at very reasonable prices
from his website
. Proceeds of the sale go to support the work of his students.

Ed is an old friend from my SF bay area
days. He is whip-smart technically, but kind and generous with his
time and efforts. (His antennas are patented, but he allows hams to
build them for their own use). He is a wonderful individual in every
way.

I own several DBJ-1's, which are
available in both amateur radio (146/446 mHz) and commercial (152/457
mHz) versions. The antennas ship with everything except the PVC pipe
to house them, and finding the right “Schedule 200” PVC can be
tricky. The much-more-common “Schedule 40” PVC is not
rf-transparent enough for the job.

Many of us would like to have large
steel towers to support our antennas, but, like network analyzers,
few of us have them sitting around. Trees, though, are much more
common. After running a DBJ-1 on the back deck of my home for some
time, it occurred to me that it could be hung from a tree, as wire
antennas are.

An early DBJ-1 iteration at NO6E broke
when the 20' PVC mast supporting it (not recommended!) toppled over.
So, while the antenna could certainly be mounted to a tree, it was
not quite sturdy enough on its own to survive long there.

My solution was to cable-tie the
antenna to a piece of “one-by-two” lumber to provide a convenient
hanging point, cable strain relief and stiffening:

The board is ordinary pine 1x2, painted
with white primer for waterproofing and to match the color of the
DBJ-1. (All the Schedule 200 pipe I've found has been white).

Since the antenna is around 5 feet
long, I used a 6 foot piece of wood. Cable ties (black for better
UV resistance) secure the antenna to the board. Note that the end
caps on the pipe are slightly larger diameter than the pipe itself,
so don't overtighten the ties immediately adjacent to the caps. (If
you buy the antenna kit, Ed supplies both caps, one with the coax
connector pre-mounted).

At the top of the board, a 3/8” (9
mm) hole allows me to attach the rope support. For the support
itself, I use parachute cord ("Paracord"),
which will support a couple of hundred pounds and is inexpensive,
especially in large spools

Cable ties are spaced every 18” (45 cm) or so. At the
bottom, connect your feedline before attaching the antenna to the
board and weatherproof the connection with your choice of tape,
sealant or whatever.

Add a couple of additional cable ties below the antenna
to secure the feedline to the board. This is especially important
when one is hanging a fair length of coax off the antenna, since coax
connectors do not provide strain relief. The ties support the cable:

The last step is hoisting the antenna into place. I
chose a long-leaf maple near the rear deck of our house. Here's the
assembly ready for hoisting:

When fully lifted, the antenna is perhaps 40' (12m) up
in the tree:

It won't come a shock to readers that hams are a
persnickety lot. In anticipation of objections and questions, I've
prepared a few replies:

Q: How do you know that the pine is rf-transparent?

A: I don't, and it likely isn't. The antenna works a
lot better than it did lower down, though.

Q: And the tree? Doesn't it absorb some of the rf?

A: I'm sure it does.

Q: Won't the cable ties deteriorate in time and need to
be replaced?

A: Yup.

Q: How the, uh, heck, did you get the paracord over the
tree branch for hoisting?

A: With
a tennis
ball cannon.
Powered by flammable hairspray, these probably-not-too-safe things
will toss a tennis ball, trailing, say, 15 pound (7 kg) test fishing
line behind it, hundreds of feet. Once the fishing line is over the
tree branch, tie the paracord to it and pull it back.

Q: Do you vouch for the safety of those launchers?

A: Absolutely not. I don't recommend using one. If you
choose to, you're on your own.

Q: What's that body of water in a couple of the
pictures?

A: Evans Creek, in Rogue River, Oregon.

Q: Is Dr. Fong paying you or asking you to write this?

A: No. He doesn't know about it.

For me, amateur radio is about making do. Sometimes,
your work will be 100% mil-spec impeccable. Mine never is. The idea
is to have fun, and I do. I always ask myself, “How good does this
NEED to be?”